Oahu Tonemaster-Hum and 21khz distortion

[wsscott]

Shooter-I made the changes shown in your last schematic, and SIGNIFICANT PROGRESS!

I can control the MIC Vol. with the Tone Pot.  Turning the Tone Pot for the MIC channel as it is now wired, adjusts the Volume of the Channel and the Treble.  It seems to have a lot more Treble output.

The MIC Vol pot seems to have no effect, I assume since it no longer has any Input source.

Is there a way that I can connect that pot so that it is functional?

I ask since it will probably be confusing to the player when he wants to increase Volume when he's using the Mic channel and nothing happens when he turns the Vol. pot.

I also can control the Instrument channel as before with its Vol Pot and also the Tone Pot.  And it too seems to have a lot more Treble.

The hum is very low, but does have a higher pitch, ie. its "brighter", as the Tone pot level is increased.

[shooter]

Is the tone pot now wired like reply 96????
is the MIC-Vol pot and .006 cap still disconnected???

[wsscott]

Yes, its wire like the schematic in 96.

Yes, the MIC-VOl pot is disconnected from the Mic Input Jack,  and the .006 cap that was part of that circuit has been removed.

[shooter]

you could move that pot to Mic-VOL and just eliminate "MIC" to help future guitarists not get confused.


if you still want a tone knob build this TS in place of the one that became "Amp volume" (reply 96)


it's from a Fender 5E3


NOTE: caps are real sensitive to hot solder pencils, you over-heat them, they do weird things like you've just experienced

[wsscott]

So putting that Mic Vol pot back into the circuit to control the amount of signal coming out of its Plate won't work? 

Not even if the Pot is placed between the 50K grid stopper resistor and the grid, or between the plate, through a coupling cap, then to the Pot, and from there to the jumper connecting the 2 plates?

[shooter]

you have a vol knob  on whatever is plugged into Mic, this is no longer the 1940's!!!!


put a NEW pot there and see what happens:)


find any guitar amp built since 1960 and copy it's volume before 1st gain stage, I'll wait 


or simply re-name the holes and call it a day, it's 18:20 here so it's herbal-tea time

[wsscott]

Fair comment.  Thanks so much for your help. Much appreciated! Best.

[wsscott]

If we find that this MIC channel configuration is just not what the owner wants/likes, etc., and since the Instrument Channel functions properly, is there an easy way to wire the circuit to just use the one active Instrument channel through both 1/2's of the tube, and maybe get more gain than it has when only just 1/2 the tube is used for the Instrument channel?

I assume I couldn't just jump the grids?

[wsscott]

I guess a Fender 5F1 Champ pre-amp circuit is really all this would be.  Pretty basic.

[wsscott]

I took the amp over to my buddy's today so he could see and hear the finished product.  He was really excited with it and loves the "crank" its got through the MIC channel.  He said to pass on his appreciation to you guys for all of your help in helping me get this amp "cranking" again.  Best.

[wsscott]

Passaloutre:  In post #93 you said:

You'd have to separate the plates. Give each one its own load resistor (double the shared value). Send the signal out from the mic channel plate through a coupling cap (say 0.05uf) to the top of its volume pot. Then send the signal from the wiper to the grid of the PI through a 220k mixing resistor.

On the instrument channel, go from the plate through a coupling cap (0.05uf) through a 220k mixing resistor to the PI grid.

If I wanted to make this Mod to the Mic channel, which grid of the PI should I connect it to, Grid A, Pin 4, or Grid B, Pin1?  Before the plates were separated they were jumped and connected to the Tone pot which connect to Grid A, Pin 4.

Thanks.

[passaloutre]

Definitely pin 4. You will want the gain provided by that stage. This would also be a good opportunity to put the instrument channel volume control after the first stage.  Again, if this were my project, I would just copy the control circuit from a 5E3 Deluxe.

[wsscott]

I think the 220K mixing resistor may be too big.  I'm not getting any volume out of that MIC Volume pot until it's around a value of 9 out of 10.  Before that really none. And there's still some influence coming from the Tone pot.  Remember that Tone Pot is also connected to Pin 4. Much less gain than the Instrument Channel and it's volume pot.

[passaloutre]

Could you show us a diagram of how you have it wired? From the discussion, it's hard to tell exactly what's what anymore. Does the tone pot still ahve the cap to ground, or is it wired as a volume pot as earlier suggested? Please draw the circuit

[wsscott]

It's not pretty, but here's my drawing of the circuit as it now exists.  The cathodes were separated and the original single 3K resistor was replaced with two 6K resistors to get the proper voltages.  The Plates on the 6SN7 preamp tube have also been separated and wired separately to the B+.  The single 100K resistor was replaced with a 168K (ie. 100k in series with 68K) resistor coming from each lead off the B+ to the Plates to get the proper voltages.  Then your Mod was added.  A .05uf coupling cap coming from the Plate on Pin 5, then to the top end of the 1M MIC Vol Pot, then wafer to 220K resistor to Pin 4 on PI, and finally bottom lead of this pot to Ground.

Hope this makes sense--at least my explanation.

[passaloutre]

6k seems a cold bias for the input tube

Does the instrument channel have its own mixing resistor?

Looking at the original schematic, does pin 4 on the PI still have a 500k grid leak to ground? You can remove that, as it makes a 2/3 voltage divider with the mixing resistor. Your volume pot now provides a grid leak.

[wsscott]

Here's a copy of the original schematic.

Is the Mixing resistor the 500K between the Tone Pot's wafer's connection and the Grid Pin 4 of the PI?

Remember, the bias resistor was originally a 3K shared between the two Cathodes.  And when we split the cathodes, we needed to double the value of the new resistors.  Originally Shooter said it should be two 1.5K resistors but I wasn't getting any real Volume, and I think you said it needed to be doubled in value not halved since the two cathodes are no longer sharing the resistor.

[scstill]

Agree about the bias.
The datasheet suggests about 3k for each triode of the 6SN7.
What is the cathode voltage with 6k?
What are you measuring at the plate?

[wsscott]

The cathodes on the 6SN7 are reading 4.06 VDC, and the Plates 88 VDC.

The Schematic show 3 VDC for the cathodes, and 75 VDC for the Plates.

[shooter]

here's the quick-e as I know it.  maybe color in what you've done since it was working and the owner happy

[wsscott]

Attached file "WES Revisions" shows the revisions as they exist now.

Note that when the cathodes were separated early on in this process, the single 3K resistor was initially changed to two 1.5K resistors, and then when the voltage wasn't correct, they were both increased to the current 6.6K.  Also, at the time that the cathodes were separated, another cap was installed with the new resistor as shown.  Both of these caps are actually 25uf/50V vs. 10uf/25V.  So each leg of the cathodes has a cap and resistor bridged.

[passaloutre]

Put a 220k resistor in seriews with the 0.05 cap coming from the instrument channel anode, and remove the 500k resistor to ground.

If that doesn't do it for you then I'll repeat my prior advice: copy the entire pre-PI circuit from the 5E3. You're already halfway there. It is a known good circuit and a better design than the Oahu or the mishmash presented below... Personally I would also use a 6SL7 in place of the 6SN7, but that's player's choice.

[wsscott]

Do you have a recommendation on where to put the 220K, ie. between the .05 cap and the wiper connection, or further on down the line?

[passaloutre]

Put it exactly where I've shown it on the diagram: between the 0.05 cap and the grid of the PI.

One end of the resistor should connect to the 0.05 cap and nothing else. The other end is connected to the node sharing the other 220k mixing resistor, the wiper of the tone pot, and the grid of the PI.

One more suggestion: the tone cap (the one between the tone pot and ground), change to 0.005.

[wsscott]

Thanks.  Sorry, I didn't realize you had made the insert on the schematic. 

[wsscott]

Sounds the best yet.  Much quieter on the "hum" too.  The Gain in the Mic channel is back where I think it should be, and there is much more control of the Tone in that Channel than before.  So this is definitely a major improvement.

What got me started with this change after my buddy heard it and liked it, was his desire to see if the MIC channel could be made just a Volume only channel, with no Tone control on it.

He plays in a group, and plays lap steel.  He also plays a standard electric guitar on some of the songs.  So he was hoping he could use the Instrument Channel for his standard guitar and use the Tone Pot with it to adjust the Volume and Tone of the guitar, BUT when he's playing the Lap Steel he didn't want to have to be changing the Tone settings that he had setup and was using for his guitar and would just be using the Lap Steel's tone controls to adjust it.  It would be easier switching between songs so he wouldn't have to fool around with changing the Tone pot settings.

Is there any way that can be achieved with this amp?

[passaloutre]

Connect the wiper of the tone pot to the wiper of the instrument channel volume pot

[wsscott]

Sorry, but that doesn't work.

I didn't change anything else, just added the jumper between the wipers of the Tone and the Instrument Vol pots.

Only can get Volume on Instrument channel when the Vol is turned almost all the way up, and nothing at all when the MIC channel is being used.

[passaloutre]

Not a jumper. Disconnect the tone pot from the grid of the PI and then connect it to the volume pot. See attached.

I'd strongly recommend using a 0.015uf for the tone cap here. The original 0.05uf may be way too dark depending on the guitar pickups.

For the record, these controls would still work better if they were after the first stage, not before. Where they are is simply duplicating the knobs *already on the guitar*

[passaloutre]

This would be a better design and fulfill the needs you have described without changing the tone of the amp.. LTSpice doesn't have a potentiometer symbol, but those doubled up 500k resistors represent 1meg pots.

[wsscott]

Got your MODS working!!  Thanks so much.

We had talked about replacing the 6SN7 with a 6SL7, so that may be something my buddy tries.  Should have a lot more gain than the 6SN7.

We'll play with this circuit for awhile, and see if he wants to go with the other schematic you included or is happy with this setup.

Best.

[passaloutre]

Glad you got it sorted out!

[wsscott]

I checked the voltages on the amp and also checked the bias for the 6V6S output tubes.  I think it's running a bit cold on the bias, but I thought I would post the numbers and get your input.

6V6S is a 14 Watt Tube.

I'm reading 36mA for each tube.

The Plate to Cathode voltage is 250.1VDC.

The Plate Voltage is 268VDC.

The Screen Voltage is 274VDC.

The Bias resistor is 200.8 ohms, with a voltage drop of 16VDC.

Rob Robinette's Bias calculator says that for a Cathode Biased Amp the safe max is 56mA.  And the plate dissipation per tube is 9 Watts at about 66%.

So do you think I should adjust the Cathode 200 ohm resistor to increase the bias?

[passaloutre]

You can try something like a 180 or 150 ohm resistor, but a lot of times the tube "self-adjusts" to the new conditions and ends up in the same place. You might think it's simple as a smaller resistor will drop less voltage, but it also increases the current through the tube so the smaller resistor drops more voltage and the dissipation ends up being not very different.

Personally, I'd leave it as is. 200 ohm is already smaller than what most 2x6V6 amps use. The power tubes should sound good and happily last a very long time.

If you're looking for more volume, there are better ways than trying to squeeze out a couple more watts from the power amp. Try a more efficient speaker.

[wsscott]

Thanks.  I was concerned it was way too cool, but I'll take your advice and leave it as is.

[passaloutre]

Just curious, do you have any more photos of the amp? I'm a big fan of these ancient beasts

[wsscott]

I'll have my buddy take a picture when its installed in the cabinet.

Here are a couple at the beginning before anything major was changed.

[wsscott]

I was wondering if the bias current is so small because the 6SN7 preamp tube maybe doesn't draw much current and have much gain.  If we put in the 6SL7 which has more than 3 times the gain that the N7 has, would this affect the bias current?  (Sorry if my terminology is not correct).

[shooter]

this comes in handy when setting up a tube, usually you can find a datasheet that has one for whatever tube you're interested in.


Gain by itself means little;


how much signal can the "next" stage take before it starts clipping?
how clean/dirty do you want the amp?
is the tube more or less "sensitive" to next-stage loading/impedance matching?


those are question you want to have BEFORE you just throw gain at the next stage

[wsscott]

Still fiddling with the amp, and a 60hz hum at low level.

I've changed some of the grounding and that improved the hum, which is dramatically better than when I started with this.  I would just like to get it a little lower.  The hum starts after the tubes warm up.

It goes dead silent when I remove V1, the 6SL7 pre-amp tube.  So does this tell me that the hum is originating somewhere between that tube and the Input jacks?  I've actually unwired the Input jacks from their pots and the hum remains.

The PT has a center tap for the rectifier tube.  I haven't added any 100 ohm resistors to the filament supply.

Any suggestions on what to try?  Thanks.

[scstill]

addressed this in reply #6 and #7
did you do these things yet?

[shooter]

So does this tell me that the hum is originating somewhere between that tube and the Input jacks?

yes.


If you know it's 60hz for sure, that is most likely filament induced hum.  so ANY signal wire that gets "close" to the fil. wires will couple the 60hz into the signal path.  the closer to the input, the more sensitive the signal path.
it can be loose tube sockets, poor solder connections, sketchy coupling caps besides just wire positioning.


are the filament wires 2 wires n a CT?  or just 2 wires no CT?
If 2 wire, do BOTH wires run to all the sockets?  or is only 1 wire run to the sockets with the other grounded??

[wsscott]

The amp is currently wired as original.  No changes to that wiring have been made.

No input signal is being injected into the "Switched" Input jacks. The jacks have been wired between their respective ground and switch lugs.

Both the main 300VDC secondary windings and the 6.3 VAC filament windings have Center Taps.

Both of the CT's connect at the same ungrounded lug on a terminal strip, and then go from there to the ground lug on the Filter Cap Can.

The 120hz hum is substantially reduced and based on a frequency meter it shows less amplitude than the 60hz hum which is at about -72db.

If I have the amp upside down so the inside is open and facing up, and if I move my hand around over it, when my hand is over the area where the Input jacks, Volume pots, and Tone pot are located, the hum is reduced.  When I move my hand away it increases.

Also I notice that the V1 Grid pin socket is loose in the socket but the tube is firmly seated in that socket.

[shooter]

If I have the amp upside down so the inside is open and facing up, and if I move my hand around over it, when my hand is over the area where the Input jacks, Volume pots, and Tone pot are located, the hum is reduced.  When I move my hand away it increases.

good indication of a crappy ground^^^

Also I notice that the V1 Grid pin socket is loose in the socket but the tube is firmly seated in that socket.

ground the pin for testing, hum change???

[wsscott]

I grounded the grid on V1 Pin1 by connecting a jumper from it to the chassis, but no difference in hum.

I also ran a shielded cable from the wiper on the Volume Pot to the Wiper on the Tone Pot, with the shield grounded on the GND lug on the Volume Pot.  That resulted in a slightly lower hum with no reaction to my moving my hand in the area.

I'm about out of grounds to test.

[wsscott]

I injected a 1Khz, 150mVrms signal into the amp, connected to a dummy load, and then connected to the scope.

From the input jack to the grid of the Output tubes it was a clean sine wave signal for both Inputs.

I found that moving around the shielded cable that I just installed between the Vol. Pot Wiper and the Tone Pot Wiper seems to clear up the hum.  There's still some background noise, but that may just be because it's "a tube amp".  I don't know.

I'll keep working on it a bit more.

[wsscott]

I forgot to mention that I also hooked the scope up to the speaker output jack, and it also has a very clean sine wave at 1Khz.  It is oscillating slowly, ie. sort of like a wave.  But the peaks don't change in amplitude, just the whole wave form rolls a bit.

If I can hear the noise/hum through the speaker, why isn't the output signal showing a sine wave at a frequency higher than the 1Khz signal that's being injected?

[shooter]

the whole wave form rolls a bit.

that's an indication of your "60hz" riding on the signal.  change your time-base and you should be able to see the lower frequency.  If your scope does FFT ..spectrum, then you should be able to see both the 1K and lower frequency together

[wsscott]

Yes the scope has FFT.  I really don't know how to use it, but here is a photo of the signal at the output speaker jack and it shows 60hz.

Now is there a way to trace it to the source of the problem?

[shooter]

yep, "walk it back", normally we follow the signal back to the input.  the problem, I suspect you'll find it "everywhere", not just along the signal path.


start by grounding your scope to the amp chassis, then probe all the grounds at there "destination"  so probe the NEG terminals of your E-caps, neg wires at pots, any ground wires at tube sockets, input jacks.


In an ideal world there will be NO AC - 60hz on any of those points.


IS this amp a 3-prong power cord????